Why guys cant tell he truth?

My boyfriend went to a function last week friday and when ne came back there was photos of a girl I dont know in his phone, when I asked him he told me its his brother's girlfriend, his brother's phone got some problems can not store pics and he asked him to store his girlfriend photos. Suddenly there was a girl calling my boyfriend now and then. When I asked him, he said its a guy that was calling. I kept quiet i took my phone i cald that number a girl responded, I nearly fainted. I asked her name she told me, It was the same name as the girl on the photos but as a contact on my man's phone was a guy name. My boyfriend wasnt with me the time i CALD this girl. I cald him asking him about all that he told me a lot of stories that its his brothers girlfriend. I kept on pushing him to tell the truth coz I could see that he is lieing to me. At he end he told me the girl was after him and his friends at the funcion took his phone and he couldnt see these pics. His friends know about them. I cald them they agreed. Its clear he is cheatin, right? What you think I should do to get the truth that he can not run away from? I LOVE HIM very much i dont wanna lose him but If realy he is cheatin, I cant stand that but do you think I should leave him? he claims that he loves me and he neva cheated on me, he can not stay without me blah blah blah u girls know how them guys moan. Plz guys help how do I make him feel pain if he is really cheating on me?